A couple of family friends have recently acquired puppies and not only does it make me nostalgic for when our dogs were puppies but you realise how unbelievably cute they were at that age. It also makes you remember how responsible you are for them, in a way, just like your own children.
Now, it would be heartbreaking for those loving pet owners amongst us if we learnt that something we had done or used had inadvertently caused harm to our pets.
Just such a case is reported in a recent edition of Clinical and Experimental Dermatology.
[ Khoo AA, Mulgrew S, Norton S. A dog’s life: an unfamiliar and lethal side effect of topical 5-fluorouracil. Clin Exp Dermatol 2018;43:718-737.]
We commonly see patients in dermatology with solar or actinic keratoses (sun induced crusty spots). These are pre-cancerous lesions that we treat because there is a small risk that some may evolve into skin cancers.
What is the treatment?
They can be treated with liquid nitrogen (or cryotherapy), removed by surgery or treated with creams such as 5-fluorouracil (5-FU) or imiquimod. One dermatologist may well see at least 50 actinic keratoses a day.
In the case above, the patient had been applying 5-fluorouracil cream (Efudix®) to actinic keratoses on his forehead. He had woken one morning to find his Jack Russell terrier licking his forehead. Soon after, the dog started vomiting, trembling and walking unsteadily. The dog was eventually euthanased.
Why does this not occur to humans?
The amount that is absorbed through the skin in humans is minimal in the vast majority of the population.
On the other hand, 5-FU is quite toxic to animals. It is more toxic to cats but there are more reported cases in dogs. Toxic doses at 5mg per kg body weight have been reported and as animals are smaller (the Jack Russell in the case report weighed 7kg), smaller doses can be toxic.
What are the signs of toxicity in animals?
These are reported as being sick (vomiting), diarrhoea, lethargy, tremors and seizures, being unsteady on their feet (ataxia), depressed breathing and heart rate abnormalities. All of these can occur within 60 minutes of exposure with death occurring up to 16 hours later.
[Dorman DC, Coddington KA, Richardson RC. 5-Fluorouracil toxicosis in the dog. J Vet Intern Med 1990;4:254-7.]
Why was the patient not warned about this?
To be honest, it is not something dermatologists are very much aware about with one report in the dermatology literature in the last 8 years.
[Snavely NR, Snavely DA, Wilson BB. Toxic effects of fluorouracil cream ingestion on dogs and cats. Arch Dermatol 2010;146:1195-6.]
However, it is something that is reported quite frequently in the veterinary literature. There are cases of death of pets after accidental ingestion of 5-FU such as after biting a tube of ointment, licking areas of skin under treatment (as in this case) or even grooming fur that has been petted by an owner with 5-FU on their hands.
[Friedenberg SG, Brooks AC, Monnig AA et al. Successful treatment of a dog with massive 5-fluorouracil toxicosis. J Vet Emerg Crit Care (San Antonio) 2013;23:643-7.]
[Sayre RS, Barr JW, Bailey EM. Accidental and experimentally induced 5-fluorouracil toxicity in dogs. J Vet Emerg Crit Care (San Antonio) 2012;22:545-9.]
[Fry MM, Forman MA. 5-fluorouracil toxicity with severe bone marrow suppression in a dog. Vet Hum Toxicol. 2004;46:178-80.]
So it is a learning point for all of us and something that I definitely counsel my patients about before handing over the prescription.
Dr Sandy Flann, Consultant Dermatologist